I think your understanding is correct.
Generally speaking, B determins A.
B is the condition that triggers A's happening.
If B happens, then A will happen. Positive or negative is not that important.:-)
-- by 会员 imstephanie (2011/3/23 11:07:36)
LZ, you got half right.
A unless B. 意思是B发生,A就不发生 A -- > neg B
Not A unless B 意思除非B发生,A才发生 A -- > B
The first is worng.
The second is right.
A unless B: Not A --> B; or Not B --> A
Not A unless B: A--> B; or Not B --> Not A
-- by 会员 sdcar2010 (2011/3/23 11:22:31)